Today, electricity is becoming one of the most popular energy sources for green transportation. Cars, trains, bicycles, boats, tramways, buses are all turning to battery powered drive train. Electrochemical accumulators (batteries) are usually considered the most practical means of energy storage and in today's transport vehicles, they are leading the market. Batteries are often composed of a string of individual electrochemical cells, also called electrochemical accumulators for the purpose of this invention. As can be understood by a person skilled in the art, electrochemical cells or accumulators include a wide variety of accumulators, including, without restrictions, lead-acid batteries, nickel-metal hydride batteries, lithium ion batteries and energy accumulating capacitors.
Depending on its chemical and atomic constitution, the voltage of an electrochemical cell is often comprised within the range of 1 to 4 volts. In the transportation domain, power requirements are very high and would imply destructive currents flowing through electrical components at such voltage (P=V*I). The transportation industry usually counters this problem by connecting multiple cells in a serial fashion thus increasing the voltage and reducing the current requirements to an acceptable level. In such a configuration, the total current circulating in the battery (string of cells) is the same in every cell.
A cell can be conceptualized as an ideal cell with a serial internal resistor. Any current flowing through this arrangement creates heat losses (P=R*I2). Since all cells are unique due to their exact atomic structure, their own internal defects and because of limitations to the manufacturing processes accuracy, their electrical characteristics are unique as well. Therefore, each cell has its own capacity, voltage and equivalent internal resistor.
This irregularity raises many problems in batteries (string of cells): for a determined current, each cell won't show the same heat losses; the full charge voltage is different for each cell; the capacity is different for each cell; the rate of charge or discharge is different for each cell. Moreover, failure of a single cell can result in a general failure of the battery.
For example, in applications related to the transportation field, electrical loads fluctuate greatly in terms of energy drained or generated during acceleration and braking. Typically, in a hybrid vehicle, the load on the battery comes from an electric traction motor that shows dynamic behavior such that average power requirement is a fraction of the peak power in transient situations. For example, the acceleration of a small car will drain a peak power in the range of 25 kW, maintaining a constant speed drain of 6 kW and braking generates 12 kW. For a car battery pack with voltage around 200 volts, the current associated with peak demands is around 125 amps. Due to their internal irregularities, such current in batteries will result in wide spread performance and electrical characteristics within each cell.
The individual electrical characteristics of each cell define its optimal operating conditions, which varies from cell to cell. To maximize the life span of each cell, it is thus preferable to operate each cell, or alternatively a plurality of groups of cells, near their individual optimal operating conditions. In each case, cells or group of cells will have maximal desired charge and discharge rates in order to maximize their life span as a function of the application for which the battery is used.
It is well known that a flywheel can be used to temporarily store energy surges, for example in case of recuperative dynamic braking of gas-electric vehicles. The energy from the flywheel can then be gradually released to a battery via an alternator. Conversely, it is also well known that a flywheel which has accumulated kinetic energy from a motor fed by batteries can rapidly release such kinetic energy, for example during the acceleration of a vehicle. For example, in a hybrid gas-electric vehicle, a flywheel allows one to reduce peaks of current intensity in the charge cycles (during recuperative dynamic braking) and discharge cycles (during acceleration) of the battery. In such cases, the flywheel effectively dampens the peaks of the charge and discharge cycles of the system.
However, one of the problems is that it is difficult to operate batteries in conditions where each individual cell is operated near its optimal conditions. Another difficulty is to provide for a method to equalize the charges between cells of a battery in the event that one or several of the cells have accumulated a charge above or below the average for the cells of the battery. In general, in the prior art, another problem arises when cells are connected in a serial fashion, which is failure of a single cell results in general failure of the whole battery.